1. Field of the Invention
The present invention relates to a light source device capable of switching the angle range of illuminating light; to a display device capable of switching the angle range of visibility using this light source device; and to a terminal device equipped with this display device; wherein the light source device, display device, and terminal device have excellent directivity of light when the angle range of illuminating light is made narrow.
2. Description of the Related Art
Due to recent technological development, display panels are deployed and used in various locations in a range of devices that includes monitors, televisions, and other large terminal devices; notebook PCs, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance), mobile phones, mobile gaming devices, and other small terminal devices. Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals in particular are deployed in a large number of terminal devices.
The modes of liquid crystal panels most often used conventionally include an STN (Super Twisted Nematic) mode using a simple matrix structure, and a TN (Twisted Nematic) mode using an active matrix structure. However, liquid crystal panels that use these modes have a narrow angle range in which gradation can be correctly distinguished, and grayscale inversion occurs outside the optimum viewing position.
This problem of grayscale inversion was relatively insignificant in small-sized terminal devices whose display content consists mainly of text. However, with recent technological developments, mid-sized terminal devices as well as small-sized mobile terminal devices have come to display not only text information, but also large amounts of graphical information. Grayscale inversion has therefore become a significant problem. Liquid crystal panels that use a mode having a wide viewing angle range in which contrast can be correctly distinguished without the occurrence of grayscale inversion are therefore gradually being installed in small-sized mobile terminal devices. Liquid crystal panels having this type of mode are referred to generically as wide-angle-view liquid crystal panels, and horizontal field modes such as In-Plane Switching systems, multi-domain vertical alignment modes, and the like are implemented therein. Since gradation can be correctly distinguished over a wide viewing angle range by using these wide-angle-view liquid crystal panels, even though a mobile terminal device is basically a personal tool, applications for sharing information with others that can be appreciated by several people simultaneously are gradually being developed and installed.
Among these terminal devices, the nature of medium-sized terminal devices in particular is such that they are used not only in closed rooms under tight security, but also in public places. It then becomes necessary to have security measures to keep displays of private information and confidential information from being viewed by a third party. Particularly in recent years, occasions where private information and confidential information are displayed in mobile terminals have increased in conjunction with progress in terminal devices, and demand for techniques to prevent surreptitious viewing is increasing. Proposals have therefore been made of an optical member for preventing surreptitious viewing, and of a display device in which the range of visible display angles is narrowed so that the display is visible only to a user who is positioned in front of the display or in another specified direction, and the display cannot be viewed from other directions.
As described above, a display having a wide viewing angle range to allow viewing by several people simultaneously, and a display having a narrow viewing angle range that can be viewed only by the user, are both individually desirable, and the ability to switch between these two types of display in a single mobile terminal device is also desirable. A liquid crystal display device has therefore been proposed in order to satisfy such requirements.
FIG. 1 is a schematic sectional view showing a first conventional viewing-angle-controlled liquid crystal display device described in Japanese Patent No. 3271695. FIG. 2 is a perspective view showing the diffusing light-guide plate used in the backlight light source of this conventional example. In the first viewing-angle-controlled liquid crystal display device as shown in FIG. 1, a liquid crystal display element 5004 is provided, a polarizing plate 5012 is provided to the front surface and to the back surface of the liquid crystal display element 5004, and a backlight light source is provided to the back surface of the liquid crystal display element 5004. The backlight light source has a two-layer structure in which an ordinary backlight light source is layered with a backlight light source that has a diffusing light-guide plate 5001. Fluorescent tubes 5008 as light sources are provided to the surfaces on both sides of a light-guide plate 5009 in the ordinary backlight light source, a scattering sheet 5007 is provided to the front surface of the light-guide plate 5009, and a prism sheet 5006 for condensing light is furthermore provided to this front surface. Specifically, this ordinary backlight light source has exactly the same structure as a backlight light source in existence prior to filing of Japanese Patent No. 3271695, and is widely used.
The front surface of the prism sheet 5006 is also provided with an opaque slitted film 5005 that transmits light only in the frontal direction and has a visible angle range of about 10° to 20° in the horizontal direction. A diffusing light-guide plate 5001 is also provided to the front surface of the opaque slitted film 5005. Fluorescent tubes 5003 as light sources are provided to the surfaces on both sides of the diffusing light-guide plate 5001. As shown in FIG. 2, the diffusing light-guide plate 5001 is a transparent plate composed of acrylic resin, for example, and numerous minute indentations 5002 are formed on the surface thereof to create a satin finish overall. As shown in FIGS. 2 and 3, the minute indentations 5002 provided to the surface of the diffusing light-guide plate 5001 are cavities that have a wall 5002a that is perpendicular to the surface of the diffusing light-guide plate 5001.
In the first conventional viewing-angle-controlled display device thus configured as described in Japanese Patent No. 3271695, when the fluorescent tubes 5003 are OFF, and the fluorescent tubes 5008 are ON, the light emitted from the fluorescent tubes 5008 passes in sequence through the light-guide plate 5009, the scattering sheet 5007, and the prism sheet 5006. A portion of the light is cut off when passing through the opaque slitted film 5005, and the light becomes narrow-angle light that is visible at angles of 10° to 20° in the horizontal direction. This light undergoes virtually no scattering even when passing through the diffusing light-guide plate 5001 from directly below, and continues to pass through the liquid crystal display element 5004 as light whose angle of visibility in the horizontal direction is narrowed to about 40° to 50°. Therefore, light is propagated only in the frontal direction, which results in the inability to recognize an image viewed from outside the range of about 40° to 50° in the horizontal direction (narrow-angle state).
When the fluorescent tubes 5003 are ON, and the fluorescent tubes 5008 are OFF, since light is scattered by the indentations 5002 of the diffusing light-guide plate 5001, light passes through the liquid crystal display element 5004 in directions other than the frontal direction. As a result, it is possible to recognize an image viewed from a wide angle of 80° or higher in the horizontal direction (wide-angle state). It is thereby possible to switch the angle range of visibility.
However, the aforementioned conventional techniques have problems such as the following. Specifically, stray light occurs during the narrow-angle state, the luminance increases at large angles in the horizontal direction, and effects for preventing surreptitious viewing are adversely affected.
As shown in FIG. 1, in the viewing-angle-controlled display device described in Japanese Patent No. 3271695, a two-layer structure is adopted in which a backlight light source having a diffusing light-guide plate 5001 is layered on an ordinary backlight light source having an opaque slitted film 5005. The narrow-angle state is created when the back-light on the back side is turned on, i.e., when the ordinary backlight light source that has the opaque slitted film 5005 is turned on.
In the case of the narrow-angle state, light in a component perpendicularly incident on the diffusing light-guide plate, which is one of the angle components of light emitted from the opaque slitted film 5005, is emitted in the perpendicular direction without necessarily being scattered by the diffusing light-guide plate. However, components of light that are not perpendicularly incident on the diffusing light-guide plate, i.e., light that is even slightly tilted in relation to the light-incident surface, enters the perpendicular walls constituting the indentations.
FIG. 3 is a sectional view showing the diffusing light-guide plate, and the drawing shows in particular the operation observed when light is incident on a perpendicular wall constituting an indentation of the diffusing light-guide plate. As previously mentioned, a minute indentation 5002 is formed in the light-incident surface of the diffusing light-guide plate 5001, and this indentation 5002 is a cavity that has a wall 5002a perpendicular to the surface of the diffusing light-guide plate 5001. Light emitted from the opaque slitted film is narrow-angle light that is visible at angles of 10° to 20° in the horizontal direction, as previously described. The present description will focus on light propagated in a direction that is tilted 5° from the direction normal to the light-emitting surface. This light enters the diffusing light-guide plate 5001 at an angle that is tilted 5° from the direction of the line normal to the light-incident surface of the diffusing light-guide plate 5001. A portion of the light incident on the diffusing light-guide plate 5001 is also incident on the wall 5002a that constitutes the indentation 5002. This light that is incident on the perpendicular wall is incident on the diffusing light-guide plate 5001 at an angle tilted 85° from the direction of a line normal to the wall. It is apparent, based on Snell's Law, that the direction of advance of light that has entered the diffusing light-guide plate is tilted 41.6° from the direction of the line normal to the wall when the index of refraction of the diffusing light-guide plate is assumed to be 1.5. This angle is 48.4° as viewed from the direction of the line normal to the light-emitting surface of the diffusing light-guide plate. Specifically, light that enters the diffusing light-guide plate 5001 from the wall 5002a that is perpendicular to the surface of the indentation 5002 is incident at an angle of 48.4° on the light-emitting surface of the diffusing light-guide plate. Since this angle is larger than the total reflectance angle of 41.8° of the diffusing light-guide plate 5001, the light is totally reflected and is propagated through the diffusing light-guide plate 5001 without being emitted from the light-emitting surface. The light propagated through the diffusing light-guide plate 5001 produces the same effects as when the fluorescent tubes 5003 provided to the diffusing light-guide plate 5001 and used for the wide-angle state are ON. Specifically, even though the wide-angle light source is turned off, the effects that prevent surreptitious viewing are reduced due to the emission of components of light that have large angles, the same as when the wide-angle light source is turned on.